The Fourth Vlba Calibrator Survey — Vcs4

This paper presents the fourth extension to the Very Long Baseline Array (VLBA) Calibrator Survey, containing 258 new sources not previously observed with very long baseline interferometry (VLBI). This survey, based on three 24 hour VLBA observing sessions, fills remaining areas on the sky above declination −40 • where the calibrator density is less than one source within a 4 • radius disk at any given direction. The share of these area was reduced from 4.6% to 1.9%. Source positions were derived from astrometric analysis of group delays determined at 2.3 and 8.6 GHz frequency bands using the Calc/Solve software package. The VCS4 catalogue of source positions, plots of correlated flux density versus projected baseline length, contour plots and fits files of naturally weighted CLEAN images, as well as calibrated visibility function files are available on the Web at 1. INTRODUCTION This work is a continuation of the project of surveying the sky for bright compact radio sources. These sources can be used as phase referencing calibrators for imaging of weak objects with very long baseline interferometry (VLBI) and as targets for space navigation, monitoring the Earth's rotation, differential astrometry and space geodesy. The method of VLBI, first proposed by Matveenko et al. (1965), allows us to determine positions of sources with nanoradian precision (1 nrad ≈ 0.2 mas). Several catalogues were compiled combining observations under various programs. The catalogue of sources observed under geodetic programs from 1979 through 2004, ICRF-Ext2 (Fey et al. 2004), contains positions of 776 sources. In addition to that, positions of 2247 sources were determined in the framework of the VLBA Calibrator Survey project: VCS1 (Beasley et al. 2002), VCS2 (Fomalont et al. 2003) and VCS3 (Petrov et al. 2005). Since 364 sources are listed in both the ICRF-Ext2 and the VCS catalogues, the total number of sources for which positions were determined with VLBI is 2659. Among them, 2269 sources, or 85%, are considered as acceptable calibrators: they had at least 8 successful observations at both X and S bands and the semi-major axis of the error ellipse of their coordinates is less than 25 nrad (5 mas). However, the sky coverage of these sources is not uniform. Successful phase referencing requires a calibrator within at